Descent Camera (Descam)
The Entry, Descent, and Landing camera (EDLcam) is mounted on the lander radar bracket and is pointed downward during lander descent. It has the exact same optical design as the Navcams and Hazcams. The EDLcam will be used to take images of Mars as the lander is descending.
The EDLcam is made of a detector head and an electronics box. The detector head houses a lens assembly and a Charge Coupled Device detector (CCD). The electronics box contains the CCD driver electronics, the 12-bit Analog to Digital Converter (ADC), camera/rover interface electronics, and a heater resistor that will keep the box above the minimum operating temperature of 218 K. The EDLcam uses a broadband visible filter and produces 1024 x 1024 pixel images.
Information in this instrument description is taken from the Mars Exploration Rover Engineering Cameras paper [MAKIETAL2003]. See this paper for more details.
The chief scientific objective of the EDLcam is:
- To acquire images of Mars as the lander descends towards the surface.
The EDLcam has been calibrated over the flight range of temperatures. It was calibrated with respect to: geometric flat field response, detector dark current, camera absolute responsivity, detector noise performance, and detector gain.
The EDLcam uses a 1024 x 2048-pixel CCD with 12 micron-square pixels and a 100% optical fill factor. The CCDs operate in frame-transfer mode, dividing the detector into two regions. One of the regions is a 1024 x 1024 pixel photosensitive imaging region where the image is recorded. The other region is a 1024 x 1024 shielded storage region where the recorded image is shifted and stored during detector readout. It takes 5.1 msec to transfer data from the imaging region to the storage region, and 5.4 seconds for readout of data from the storage region. Each CCD includes 32 non-imaging pixels in the serial readout registers, which allow the monitoring of the CCD electronics offset and detector noise performance. The RMS read noise at cold temperatures (218 K) is approximately 20 electrons, and the detector systems have gain values of approximately 50 e-/DN, which results in a system with approximately .5 DN of RMS read noise.
The absolute CCD quantum efficiency (QE) of the CCDs has been measured between 400 and 1000 nm at four operating temperatures ranging from 218 K and 278 K. The QE of the MER CCDs is typical of a silicon CCD detector, with sensitivity peaking at 700 nm with a QE value of approximately 43%. The SNR of the detector system is essentially Poisson-limited because of its low readout noise and small dark current rates in the Martian operating environments.
The EDLcam CCD uses a 'clocked antiblooming' readout technique, instead of having anti-blooming circuitry. The proper choice of autoexposure parameters prevent the blooming effect.
The EDLcam uses a broadband filter with the center at approximately 750 mn and a Full Width at Half Maximum (FWHM) of approximately 200 nm.
The EDLcam has a f/12 optical system with a 14.67 mm focal length. It has a 45 x 45 degree field of view, and a 67 degree diagonal field of view. The angular resolution at the center of the field of view is .82 mrad/pixel.
The EDLcam is mounted on the lander radar bracket.